1. Field of the Invention
The present invention relates to a Schottky barrier semiconductor device provided with electrode layers forming Schottky barriers in an SiC semiconductor layer.
2. Description of the Related Art
An SiC Schottky barrier diode is used in, for example, a high-frequency circuit owing to its switching characteristics of enabling high-speed operation and a relatively small forward loss. In a conventional SiC Schottky barrier diode, a Schottky barrier formed by joining an electrode layer to a semiconductor layer can be changed depending on a material such as a metal used in the electrode, and a forward rising voltage and a reverse leakage current change in accordance with the height of the Schottky barrier. One electrode material which increases the height of the Schottky barrier is nickel (Ni). When this material is used as the electrode material, a threshold voltage increases, but the reverse leakage current is suppressed. In contrast, one material which decreases the height of the Schottky barrier is titanium silicide (TiSi2). When this material is used, the height of the Schottky barrier decreases, but the reverse leakage current increases. Thus, the electrode material having a low forward threshold voltage increases the reverse leakage current, and it has been impossible that both the forward threshold voltage and the reverse leakage current are low.
On the other hand, there has been disclosed a structure in which a junction barrier is provided at a Schottky junction so that the reverse leakage current of a Schottky barrier semiconductor device is reduced to increase a reverse breakdown voltage (refer to JP-A 59-35183 [KOKAI]). In this Patent document, the reverse leakage current is reduced to enhance the breakdown voltage by a depletion layer which is formed on the side of a first semiconductor region (2) by a junction barrier (4), as disclosed in FIG. 1 of this document.
As described above, the Schottky barrier using a practical electrode material has the characteristics of the forward threshold voltage and the leakage current corresponding to the electrode material, and it has heretofore been impossible to avoid such reciprocal characteristics. Moreover, if a semiconductor region whose conductivity type is different from that of the above-mentioned SiC semiconductor layer serving as a drift layer is formed on the surface of this semiconductor layer, this region does not function as an operation region, and the work area of the drift layer is reduced. In this case, there is a problem of increased series resistance between the electrode layer and an ohmic electrode provided on the rear surface of an SiC semiconductor substrate. Further, the SiC Schottky barrier semiconductor device is required to ensure operation at a high temperature. Especially when the dependency of the leakage current on the temperature is considered, stronger suppression of the leakage current is needed at the high temperature, and a more advanced technique for suppressing the leakage current is demanded.
In view of the foregoing circumstances, the most satisfactory condition of combination is searched for, and it is then clear that a junction barrier controlled Schottky (JBS) structure can be used which selects a Schottky electrode material having a low work function to reduce an on-voltage and which suppresses the reverse leakage current as a measure against the leakage current. A Schottky electrode material having an excessively low work function has an excessively high reverse leakage current and leads to difficulty in obtaining a diode characteristic as such. However, if the height of the Schottky barrier is 1.2 eV or more, the threshold voltage of a conventional Si-PiN diode having the same breakdown voltage is exceeded. Thus, a sufficient reduction of loss expected by the use of the SiC diode might be impaired. It is necessary that the height of the Schottky barrier be 1 eV or less in order to provide a threshold voltage equal to at least that of the Si-PiN diode.
Thus, it has been desired to realize an SiC Schottky barrier semiconductor device which stably operates without much increase of the reverse leakage current even in the case of a temperature variation.